The present application relates to a method for producing positive lead electrodes. More particularly, it relates to a method for producing positive electrodes for lead storage batteries in which a lead core or grid serving as a conductor, is enclosed in a layer of lead dioxide (PbO.sub.2).
Positive lead storage battery electrodes can be divided into two main classes, the so-called tubular electrodes, and the so-called "pasted" electrodes. Tubular electrodes are built up on lead grids consisting of a number of rods protruding from a combining strip. Around each rod there is placed a sheath, and the active material is charged into the space between the sheath and the rod. The tube is thereafter closed in a suitable way. When it is charged into the spaces between the sheath and the rod, the active material must be in the form of a dry powder with good rheological properties. Pasted electrodes are built up on a lead grid which is generally in some checkerboard form. Starting with a lead oxide of essentially the same kind used in the manufacture of tube electrodes, a paste is produced by the addition of suitable amounts of water and sulfuric acid, and possibly other additives. The mass thus prepared is then applied to the lead grid, either manually or automatically.
No matter what the type of electrode in question, it must be subjected to a so-called "forming" step in a subsequent manufacturing procedure. This "forming" is the initial charge of the electrodes, whereby substantially all of the active material is changed to lead dioxide. Forming is of great importance for the future properties of the electrode, both with respect to its capacity and to its life expectancy.
Three essential problems arise in the "forming" step, however, which are of great significance for the quality of the finished electrode. The active material that is delivered as a fine-grain powder has no mechanical strength, and the electrodes therefore are sensitive to mechanical stress. They have to be handled with great caution. Besides, the pulverous material has a certain tendency to work out through the porous sheath. If too much material is thus removed from the electrode, this obviously affects the quality; but, above all, it creates a problem with respect to the environment because the powder, which primarily consists of various lead oxides, is very poisonous. Finally, various modifications of the lead dioxide can be produced during the "forming" so that it is essential that "forming" occur in the most favorable circumstances possible for the production of the preferred form of lead dioxide.
"Forming" is a conventional procedure and is well known to those skilled in the art of manufacturing lead storage batteries. In substantially all methods thus far known, the forming has been effected in sulfuric acid electrolyte. The forming electrolyte generally has a lower density than the electrolyte that is intended for use in service in the finished battery. A customary elecctrolyte density for forming is 1.10-1.15 gram per cc.
It has been known that forming can be conducted in electrolytes other than sulfuric acid. One such electrolyte used in forming positive electrodes is an ammonium sulfate solution. However, this electrolyte has been regarded as one that entails special complications for positive electrodes. A method is also known for forming positive electrodes in two steps, in different alkaline electrolytes. Here, a weakly alkaline electrolyte is used in the first step, and in the second, a strongly alkaline electrolyte. This obviously involves complications in manufacture since it requires an additional step and all of the equipment needed to perform it.
In forming, the effort is made, on the one hand, to obtain good electrical contact between the lead conductor and the active material, and, on the other hand, to have good mechanical strength as well as a favorable structure in the active material, so that the electrode will have high capacity and a long life expectancy.
The form of lead dioxide that is sought in electrode manufacture is .alpha.-PbO.sub.2. This form imparts desirable mechanical properties to the electrodes. Although electrodes with large quantities of .alpha.-PbO.sub.2 have a relatively low initial capacitance, this is of slight importance because there is a working up of the electrodes, and the electrodes with large amounts of .alpha.-PbO.sub.2 have proved to be superior in the matter of life expectancy.
One object of the present invention is to provide a method for the production of positive electrodes for use in lead storage batteries having improved electrical contact between the lead conductor and the active material.
Another object of this invention is to provide a method for the production of positive electrodes for use in lead storage batteries having improved mechanical strength.
Still another object is to provide a method whereby substantially all of the lead dioxide in the formed electrode is the .alpha.- form (.alpha.-PbO.sub.2).
A further object of the present invention is to provide a method for the production of positive lead dioxide electrodes having improved capacity as a result of high porosity.